KMEM(9) NetBSD Kernel Developer's Manual KMEM(9)
NAMEkmem -- kernel wired memory allocator
SYNOPSIS#include <sys/kmem.h>void *kmem_alloc(size_t size, km_flag_t kmflags);
void *kmem_zalloc(size_t size, km_flag_t kmflags);
voidkmem_free(void *p, size_t size);
char *kmem_asprintf(const char *fmt, ...);
options DEBUGDESCRIPTIONkmem_alloc() allocates kernel wired memory. It takes the following argu-
ments.
size Specify the size of allocation in bytes.
kmflags Either of the following:
KM_SLEEP If the allocation cannot be satisfied immediately,
sleep until enough memory is available.
KM_NOSLEEP Don't sleep. Immediately return NULL if there is
not enough memory available. It should only be used
when failure to allocate will not have harmful,
user-visible effects.
Use of KM_NOSLEEP is strongly discouraged as it cancreate transient, hard to debug failures that occurwhen the system is under memory pressure.
In situations where it is not possible to sleep, for
example because locks are held by the caller, the
code path should be restructured to allow the allo-
cation to be made in another place.
The contents of allocated memory are uninitialized.
Unlike Solaris, kmem_alloc(0, flags) is illegal.
kmem_zalloc() is the equivalent of kmem_alloc(), except that it initial-
izes the memory to zero.
kmem_asprintf() functions as the well known asprintf() function, but
allocates memory using kmem_alloc(). This routine can sleep during allo-
cation. The size of the allocated area is the length of the returned
character string, plus one (for the NUL terminator). This must be taken
into consideration when freeing the returned area with kmem_free().
kmem_free() frees kernel wired memory allocated by kmem_alloc() or
kmem_zalloc() so that it can be used for other purposes. It takes the
following arguments.
p The pointer to the memory being freed. It must be the one
returned by kmem_alloc() or kmem_zalloc().
size The size of the memory being freed, in bytes. It must be the
same as the size argument used for kmem_alloc() or kmem_zalloc()
when the memory was allocated.
Freeing NULL is illegal.
NOTES
Making KM_SLEEP allocations while holding mutexes or reader/writer locks
is discouraged, as the caller can sleep for an unbounded amount of time
in order to satisfy the allocation. This can in turn block other threads
that wish to acquire locks held by the caller. It should be noted that
kmem_free() may also block.
For some locks this is permissible or even unavoidable. For others, par-
ticularly locks that may be taken from soft interrupt context, it is a
serious problem. As a general rule it is better not to allow this type
of situation to develop. One way to circumvent the problem is to make
allocations speculative and part of a retryable sequence. For example:
retry:
/* speculative unlocked check */
if (need to allocate) {
new_item = kmem_alloc(sizeof(*new_item), KM_SLEEP);
} else {
new_item = NULL;
}
mutex_enter(lock);
/* check while holding lock for true status */
if (need to allocate) {
if (new_item == NULL) {
mutex_exit(lock);
goto retry;
}
consume(new_item);
new_item = NULL;
}
mutex_exit(lock);
if (new_item != NULL) {
/* did not use it after all */
kmem_free(new_item, sizeof(*new_item));
}
OPTIONS
Kernels compiled with the DEBUG option perform CPU intensive sanity
checks on kmem operations, and include the kmguard facility which can be
enabled at runtime.
kmguard adds additional, very high overhead runtime verification to kmem
operations. To enable it, boot the system with the -d option, which
causes the debugger to be entered early during the kernel boot process.
Issue commands such as the following:
db> w kmem_guard_depth 0t30000
db> c
This instructs kmguard to queue up to 60000 (30000*2) pages of unmapped
KVA to catch use-after-free type errors. When kmem_free() is called,
memory backing a freed item is unmapped and the kernel VA space pushed
onto a FIFO. The VA space will not be reused until another 30k items
have been freed. Until reused the kernel will catch invalid accesses and
panic with a page fault. Limitations:
· It has a severe impact on performance.
· It is best used on a 64-bit machine with lots of RAM.
· Allocations larger than PAGE_SIZE bypass the kmguard facility.
kmguard tries to catch the following types of bugs:
· Overflow at time of occurrence, by means of a guard page.
· Underflow at kmem_free(), by using a canary value.
· Invalid pointer or size passed, at kmem_free().
RETURN VALUES
On success, kmem_alloc() and kmem_zalloc() return a pointer to allocated
memory. Otherwise, NULL is returned.
CODE REFERENCES
The kmem subsystem is implemented within the file sys/kern/subr_kmem.c.
SEE ALSOintro(9), memoryallocators(9), percpu(9), pool_cache(9), uvm_km(9)CAVEATS
Neither kmem_alloc() nor kmem_free() can be used from interrupt context,
from a soft interrupt, or from a callout. Use pool_cache(9) in these
situations.
SECURITY CONSIDERATIONS
As the memory allocated by kmem_alloc() is uninitialized, it can contain
security-sensitive data left by its previous user. It is the caller's
responsibility not to expose it to the world.
NetBSD 6.0 June 14, 2011 NetBSD 6.0

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